Well known in the prior art are security systems that transmit coded signals to a central station in the event an alarm condition exists. Also known in prior art security systems is the general concept of wireless alarm monitoring where the monitored stations are responsive to a clock signal transmitted by the central station. Nowhere in the prior art, however, is there disclosed a security system that has the capability of using a pre-existing radio transmitter in either one-way or two-way configurations, nor does the prior art show the expanded signal processing capability available in this invention. Finally, no prior art system discloses a signal validating process utilizing redundant code comparison as is provided in the present invention.
U.S. Pat. No. 3,508,260 to Stein for "Transponder Monitoring System" discloses a security system having a central station, with a separate channel for each responder, and remote responder stations which regularly signal the central station in the absence of an alarm condition. A major advance of the present invention lies in its use of the signal-on-change logic, thus eliminating the rather complicated resetting method of Stein. Another advantage of the present invention rests in the use of a digitalized signal to transmit information between the central station and the remote responder stations. Finally, the Stein system is inherrently a two-way system while the present invention is not so limited.
U.S. Pat. No. 4,257,038 to Rounds et al. for "Coded Security System" discloses a security system comprising a central station and nearby signal units that transmit coded signals which indicate whether a security alarm is enabled or disabled. The device of Rounds et al. is a simple one-way communication device not intended for the complex signal processing envisioned by the present invention. The present invention, unlike that of Rounds et al., has the facility for two-way radio communication and is capable of using a pre-existing transmitter. In addition, the patent to Rounds et al. discloses a test mode which is far different from that of the present invention; it requires actual human intervention to be activated. Nothing disclosed in the patent to Rounds et al shows. test means for measuring signal validity. The security system of the present invention overcomes these disadvantages by being capable of automatically testing the viability of the remote station and measuring the validity and strength of the signal received from the remote station.
U.S. Pat. No. 3,949,397 to Wagner et al. for "Identification--Friend or Foe System and Method" discloses a process for testing the validity of a response signal. The method involves setting up a protocol which allows a response signal to be used only once during a particular time interval so that a second signal transmitted within the same time interval will be recognized as spurious. The protocol requires complicated programming that is unnecessary in the present invention's use of reduntant code comparision.
U.S. Pat. No. 4,067,008 to Sprowls for "Multiplex Interrogation System" discloses a security system which requires physically linking the central station to the remote sensing units. The Sprowls system requires the central station to address each remote sensing unit sequentially. This method is very limited and time consuming. By contrast, the present invention is capable of signaling each remote responder station directly or randomly, as well as sequentially, thus overcoming the prior art limitations. This also results in significant time savings not available in the prior art.
U.S. Pat. No. 3,336,591 to Michnick for "One-Way Range and Azimuth System for Stationkeeping" discloses a one-way system in which time clocks are associated with each remote sending unit. In the two-way configuration, the security system of the present invention dispenses with the need for timing units located at the remote responder stations. This is accomplished by the central station of the present invention which is capable of transmitting a periodic synchronizing signal interrogating the remote responder station. Finally, the present invention is capable of transmitting more varied information than is possible in the system of Michnick.